Manufacturing of fibrocement articles without asbestos fibre

0001The present invention relates to fibrocement materials and more in particular to a process for the construction of articles, such as slabs, in fibrocement without asbestos fibres.

0002On one hand, the production of slabs in asbestos cement using a mixture of cement and asbestos fibres diluted in water, usually with a water/cement ratio of approximately 10, is well established, the asbestos fibres acting as both process and reinforcement fibres.

0003On the other hand, it is known that the use of asbestos fibres, even in construction materials, is forbidden in certain countries for sanitary and ecological reasons.

0004For this reason researches have been carried out for alternative solutions to asbestos cement construction materials without asbestos fibres.

0005Different solutions to the problem have been suggested in the British patents 1,605,004, 2,012,831, 2,073,317 and in the European patent specification 0,047,158. Each of these solutions requires the realization of fibro cement materials which substantially include particular cellulose fibres as process and reinforcement components. Their use can assure that the product which is still at the plastic stage has to be pressed and stabilized in autoclaves with saturated steam at a temperature of approximately 180° and pressure of approximately 10 Atm.

0006More in particular, in the Italian patent 1.128.037 a process for the manufacturing of a hydraulically set material reinforced with fibres is claimed, characterized in that the fibres include 2 to 20% in volume, as solids, of filtering or processing fibres and 1 to 10% in volume, as solids, of reinforcement fibres. Both types of fibres are put through a pre-treatment which tends to increase the dispersion in the suspension. This pre-treatment of the fibres is designed to make at least one compound, a salt, precipitate in or on the fibres so that they are combined with a first saline compound solution and then brought in contact with a second saline compound.

0007The process is therefore laborious, onerous and in practice uses: <ul id="ul0001" list-style="dash" compact="compact"><li>filtering or process fibres which allow the use of traditional machines for manufactured articles in fibrocement reinforced with asbestos fibres; and</li><li>reinforcement fibres which grant the mechanical-physical characteristics required by the manufactured articles.</li></ul>

0008Thus, cellulose, waste paper, woolen fibres, silk, polyproylene fibres, rock wool and kaolin wool are used as process fibres and steel, carbon, glass, polypropylene, polyvinyl-alcohol, polyester, polyamides and polyacrylic are used as reinforcement fibres.

0009In each case, said fibres require a specific preliminary chemical treatment and the addition of a flocculator able to improve the retention of the solid particles for their dispersion in the mixture.

0010In practice, so as to achieve a positive result, the percentage of process and reinforcement fibres immersed in the dry mixture is usually high with respect to the one relating to asbestos fibres in articles manufactured with these fibres, even if this is not so in said Italian patent application. In particualar, these percentages are approximately 7-8% in weight for process fibres and approximately 2-3% for reinforcement fibres, which in volume correspond to 14-16% and 4-6%, respectively.

0011It should therefore be noted that the specific gravity of these fibres is about half that of asbestos which, when going in the initial dry mixture, usually is about 11%, in weight. These alternative fibres therefore give the product, due to their high volume, a very low density (about 1.2 ö 1.3 g/cm3 against 1.6 ö 2 g/cm3 of the asbestos cement), thus achieving a low bending strength after the finished product has been immersed in water, in accordance with various European Standards.

0012It is therefore necessary to work on the material being manufactured when it is still at the plastic stage with a pressing action of about 20̸0̸ - 250̸ kg/cm2 so as to bring the density to 1.5 - 1.6 g/cm3.

0013Also in GB-A-1 498 966 an article is disclosed as formed from a composition comprising a water-settable inorganic binder which is one or more of a calcium silicate binder. Portland cement, aluminous cement and blast furnace slag cement, fibrous reinforcing material comprising cellulosic fibres and mica. However this execution uses calcium silicate binder and polyamide and polypropilene fibres and it requires a setting of the binder by an autoclaving reaction or by air curing. In the documents Chemical Abstract 10̸5(1986)8 abstract n.84269s and GB 2 0̸83 512 are disclosed asbestos-free articles made from compositions comprising specific quantity of cellulosic fibres, mica, reinforcement fibres, cement and floculator. But they do not disclose any preliminary treatment and mechanical opening in a water solution of the cellulosic fibres nor a final thermal seasoning of product in a chamber at controlled temperature.

0014In addition, the results of the known techniques are not completely positive and reliable, especially because the cellulose contents (especially if in large quantities) is negative for the duration of the product and causes shrinkages of the slabs and their cracking, especially during very hot and dry weather.

0015Slabs of fibrocement which can be considered of a certain durability are those with: <ul id="ul0002" list-style="dash" compact="compact"><li>a minimum percentage of cellulose in the mixture so as to have a good retention of the cement particles during the slab forming process;</li><li>a specific gravity of the slab of approximately 1.4 g/cm3 so as to have a good resistency against freezing;</li><li>the presence in the mixture of primary substances of components to sufficiently contrast the tension of the shrinkages caused by cellulose in the mixtures, and</li><li>a particular seasoning of the product in a conditioning chamber with controlled temperature and humidity.</li></ul>

0016The object of the present invention is to solve the problem of the production of articles in fibrocement without any asbestos in a new and original way, keeping in mind the above mentioned requirements and with a new formulation of the process and reinforcement components.

0017It is in fact an object of the present invention to allow the manufacture of articles with mechanical-physical characteristics similar to those of materials containing asbestos, but also withouth coming across inconveniences, defects, limitations and unreliability of the manufactured articles proposed in the above mentioned publications and, advantageously, withouth having to carry out chemical treatments or pre-treatments, pressing operations and treatments in autoclaves.

0018Thus, in accordance with the present invention, a process is proposed for the production of shaped articles in fibrocement without asbestos fibres, which comprises: <ul id="ul0003" list-style="none"><li>(i) the formulation of a mixture with <ul id="ul0004" list-style="dash" compact="compact"><li>cellulose fibres in a quantity of 2 to 5% by weight;</li><li>a filler material selected among micron-silicon, sepiolite or kaolin powders or fly ash, in a quantity of 5 to 10̸% by weight;</li><li>reinforcement fibres selected among polyvinyl alcohol based fibres and acrylic fibres about 6 mm in lenght, in a quantity of 1 to 3% by weight;</li><li>an agent for contrasting the tension due the expansion of the finished article, selected between wollastonite and mica in the form of acidulate, in a quantity of 5 to 10̸% by weight;</li><li>Portland 325 cement as necessary;</li><li>a floculant in water;</li></ul> and after the articles have been shaped;</li><li>(ii) a final thermal conditioning for a seasoing of the articles effected by passing the article through a thermally conditioning chamber with a first area at about 30-40°C, a second area at about 50-60°C, and a final area at about 70-80°C, the treatment being carried out for a total duration of approximately 10 hours in an environment with high humidity.</li></ul>

0019The low percentage of cellulose fibres improves the durability of the manufactured articles and reduces their risk of cracking due to shrinkages or thermal expansions. A weight percentage of 2 to 5%, preferably of 3 to 4% is retained to be the most satisfactory.

0020Furthermore, in accordance with the invention, said fibres are put through a preliminary mechanical treatment so as to achieve an optimal opening of the fibres themselves through a defibrating refiners which operate on cellulose in an acqueous solution containing 4 - 5% of cellulosic fibres until it reaches a drainage value of 50̸-70̸° Shopper Riegler.

0021This drainage value is considered as being important so as to achieve a good retention of the cement particles in the Hatschek forming machine.

0022The addition of micro-silicon, sepiolite or kaoline powders or fly ash in a quantity of 5 to 10% in weight, preferably 7 to 8%, will give the layered damp material a good plasticity during the forming of the article which is necessary for its shape, especially when regarding corrugated slabs, as well as obtaining a good adhesion between the various layers of laminations, avoiding the delamination of the same as they are usually made up of various layers.

0023Polyvinyl-alcohol based fibres and/or acrylic fibres of approximately 6 mm in length to a weight of 1-3%, preferably of 1.5-2.5%, are used as reinforcement fibres.

0024Wollastonite and/or mica of the above mentioned percentages are preferably used as tension contrasting agents. Infact, said powders in the form of acidulate constitute a micro-reinforcement inside the final mass which impedes any micro-flaws in the manufactured article.

0025The Portland cement used could contain a siliceous load and on the other hand the addition of a flocculator in water further improves the filtration properties of the formulated mixture.

0026Finally, the necessary thermal conditioning of seasoning of the manufactured articles is carried out in a chamber with a controlled temperature having a first area of 30 to 40°C, a second area of 50 to 60°C and a final area of 70 to 80°C.

0027The slabs remain inside the chamber for approximately 10 hours in an enviornment with high humidity so that the curing is achieved in the best possible way obtaining reliable and long lasting manufactured articles.